Simon LeVay

Introduction

Simon LeVay (born August 28, 1943, in Oxford, England) published one of the most consequential single papers in the neuroscience of sexual orientation. The 1991 Science report — "A difference in hypothalamic structure between heterosexual and homosexual men" — was the first study to describe a structural difference in the human brain associated with sexual orientation, and it detonated far outside the field: it was discussed on PBS, featured in Newsweek, and sent LeVay onto television programmes including Donahue and Oprah within weeks of publication.

Less often repeated is what the paper actually showed and what LeVay himself has said about it ever since. He found a group difference in average volume of a structure called INAH3, with heavy overlap between groups; he reported no animal model, no developmental mechanism, and no causal claim; and he has spent thirty years saying the finding demonstrates neither that sexual orientation is genetic nor that the difference causes rather than follows from orientation. Byne's partial replication did not confirm the effect, and no one has succeeded in doing so since.

LeVay came to this work sideways. He was a vision scientist trained in the line of David Hubel and Torsten Wiesel, working on the architecture of the visual cortex. He changed fields after his partner died of AIDS, and afterwards never returned to bench neuroscience. His later career has been as a writer, teacher, and synthesizer — including a widely used human sexuality textbook now in its sixth edition.

Early Life and Education

LeVay spent most of his childhood in West Dulwich in south London, attending Dulwich Preparatory School and then Dulwich College, where he specialised in Latin, Greek, and Ancient History, and was a serious cyclist. He has written that it was at Dulwich that he acknowledged to himself that he was gay.

His route into science did not pass through a conventional sixth form. Before university he spent a year in Göttingen, Germany, working as a technician in an electron microscopy laboratory, learning German, and publishing — still as a school leaver — a paper on the ependymal cells of the chick embryo spinal cord with Paul Glees in 1964. He then read natural sciences at the University of Cambridge, graduating in 1966.

He began the clinical portion of medical training at University College Hospital in London and dropped out. Medicine's loss was anatomy's gain: he returned to Göttingen for graduate work and completed a doctorate in neuroanatomy in 1971, with a dissertation on the neurons and synapses of the lateral geniculate nucleus of the monkey and the effects of enucleating an eye. Around this time he met Richard Hersey, an American exchange student from Berkeley, and followed him to the United States when Hersey returned home. They were together for twenty-one years.

Academic and Professional Career

LeVay was a postdoctoral fellow at Harvard Medical School from 1972 to 1974, working in the laboratory of David Hubel and Torsten Wiesel during the years that laboratory was mapping the visual cortex. With them he published work on the pattern of ocular dominance columns in macaque visual cortex revealed by a reduced silver stain, and he continued the line in studies of synaptic patterns in cat and monkey visual cortex. He held positions in neurobiology at Harvard Medical School from 1974 to 1984, and received a Sloan Research Fellowship in 1975.

In 1984 he moved to the Salk Institute for Biological Studies in San Diego, with a concurrent appointment as adjunct associate professor of biology at the University of California, San Diego. He took leave to care for Hersey, who had contracted AIDS, and Hersey died in 1990. LeVay returned to Salk with, by his own account, no appetite for continuing the visual work. He said later that it was while looking after Hersey that he decided he wanted "to do something different with my life," describing an "emotional need to do something more personal, something connected with my gay identity."

He was, by training, already examining structure and function in one part of the brain, and later said the switch did not feel like a large leap. The result was the INAH3 study. It made him a public figure, and he used the position: in 1992 he took a further leave from Salk to help found the Institute of Gay and Lesbian Education in West Hollywood with Chris Patrouch and Lauren Jardine, serving as its instructor and president. The institute struggled with enrolment and with defining its academic mission, and closed in 1996. LeVay did not return to laboratory research. He became a lecturer in Human Sexuality Studies at Stanford University in 2003 and has taught and lectured there and elsewhere since.

His writing has continued into the present. Recent work includes Attraction, Love, Sex: The Inside Story (Columbia University Press, 2023), a young adult novel (2025), and successive editions of the textbook now titled Discovering Human Sexuality, which he co-authors with Janice Baldwin and John Baldwin.

Major Contributions to Psychology

The INAH3 study, in its own terms

The background was a UCLA finding that one of four interstitial nuclei of the anterior hypothalamus — INAH3 — was on average twice as large in men as in women. LeVay asked whether the dimorphism tracked sexual orientation rather than sex. Over nine months he dissected the hypothalami of forty-one cadavers from hospitals in New York and California: nineteen homosexual men, all of whom had died of AIDS-related illness; sixteen men presumed heterosexual, six of whom had also died of AIDS; and six women presumed heterosexual. (Counts are sometimes given as eighteen gay men plus one bisexual man, reflecting different ways of reporting the same sample.)

Brains were numerically coded so he was blind to donor classification during measurement. He found no difference between the groups in INAH1, INAH2, or INAH4. INAH3 in the heterosexual men was more than twice the volume of that in the homosexual men, and the difference held when the comparison was restricted to the heterosexual men who had died of AIDS. The values in the gay men were statistically indistinguishable from those in the women.

His own conclusion was careful: that INAH3 is dimorphic with sexual orientation rather than with sex, at least in men, and that this "suggests that sexual orientation has a biological substrate." Almost every subsequent problem with the study's public reception comes from dropping one of those qualifiers.

Making the topic a serious subject

The second contribution is less contested. Through The Sexual Brain (1993), Queer Science (1996), and especially Gay, Straight, and the Reason Why (covering findings from more than six hundred studies, and winner of the Bullough Award from the Foundation for the Scientific Study of Sexuality), LeVay became the person most responsible for making the biology of sexual orientation a topic with a synthesised, surveyable literature rather than a set of isolated, contested findings. He is also unusual among scientists in this area for repeatedly writing about how the research can be misused, including by his own side.

Major Publications

  • LeVay, S. (1991). A difference in hypothalamic structure between heterosexual and homosexual men. Science, 253(5023), 1034–1037.
  • LeVay, S. (1993). The Sexual Brain. MIT Press.
  • LeVay, S. (1996). Queer Science: The Use and Abuse of Research into Homosexuality. MIT Press.
  • LeVay, S. (2011). Gay, Straight, and the Reason Why: The Science of Sexual Orientation. Oxford University Press. Second edition, 2016.
  • LeVay, S., & Baldwin, J., & Baldwin, J. (2025). Discovering Human Sexuality (6th ed.). Oxford University Press.

Impact and Legacy

The replication did not land. William Byne and colleagues repeated the work with different tissue: fourteen HIV-positive homosexual men, thirty-four presumed heterosexual men (ten HIV-positive), and thirty-four presumed heterosexual women (nine HIV-positive). They confirmed the original sex difference — INAH3 was significantly larger in heterosexual men than in heterosexual women — but the homosexual group fell between the heterosexual men and the women, and was not significantly different from either. They also conducted measurements LeVay had not done: INAH3 weight showed the same pattern, while neuron counts found a sex difference and no trend whatsoever with sexual orientation. Byne's summary was blunt: that the work has not been replicated, and that human neuroanatomical studies of this kind have a poor track record for reproducibility. LeVay's reply is a technical one — that a one-tailed test was appropriate given his prior published finding, and that it would have reached significance. That is a real argument, but it is a reply about statistical convention rather than a fresh dataset.

The AIDS confound cannot be fully removed. HIV can affect the central nervous system, and essentially every gay man in the sample had died of AIDS, so critics argued that illness rather than orientation could account for the size difference. LeVay's answer is built into the paper: he did include heterosexual men who died of AIDS, and found no significant INAH3 difference between them and heterosexual men dying of other causes. J. Michael Bailey called the objection "aggravating" on exactly these grounds, and noted the further point that the hypothalamus develops early, so no expert he consulted thought the alternative causal direction — that being gay shrank the nucleus — was plausible. This is the strongest part of the defence, and it is not by itself sufficient.

Group differences do not license individual inference. The distributions overlap heavily. Anne Fausto-Sterling put it in terms no one has improved: if LeVay picked a nucleus of middling size he could not tell you whether it came from a heterosexual or a homosexual man. Ruth Hubbard and Elijah Wald made the same point at greater length. The upshot is that the result supports no claim about any individual brain.

The sample was small and partly assumed. Forty-one brains is small for this kind of measurement, especially given that these nuclei are hard to see in tissue slices and that colleagues disputed whether volume or cell count is the right measure. The sexual histories were thin: the heterosexual men were presumed heterosexual largely from the base rate, with only two men specifically recorded as denying same-sex contact. No tissue from lesbians was available at all — which means the study's own symmetrical prediction was never tested.

Subsequent evidence moved but did not settle. Twin studies, the fraternal birth order effect, and animal models provided independent lines of support for biological contributions to orientation, and roughgarden noted objections pointing out sexual dimorphism does not uniformly follow the "crossed" pattern the INAH3 result implies. Notably, Swaab's group later reported that the suprachiasmatic nucleus is larger in homosexual men, roughly twice the volume and neuron number, which does not fit a simple "gay brains are female brains" story. The broader lesson others have drawn from the affair is not about LeVay specifically: a single, unreplicated, high-profile anatomical finding can carry enormous social weight, in both directions, long after the evidence stops supporting it.

The response from both ends of the political spectrum at the time says something too. Some religious groups objected to the research; some members of LGBT communities objected as well, worried that a biological marker would invite screening or attempts at prevention. LeVay's position has been consistent: the finding describes a correlation, says nothing about individuals, and does not establish cause.

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References

  • Levay, S. (1991). A difference in hypothalamic structure between heterosexual and homosexual men. Science, 253(5023), 1034-1037.

Last updated: 2026-10-20

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